In an AP-based wireless local area network, a STA communicates with a specific AP situated in the vicinity of the STA; the STA is said to be associated to this AP. It is sometimes necessary or desirable for a STA to change the AP to which it is associated. For example, the STA may be moving out of the area covered by the AP to which it is originally associated (i.e., roaming) and experiencing poor signal conditions. Another situation is when throughput becomes poor due to congestion arising in the basic service set (BSS) served by the original AP.
In order for a STA to reassociate to a new AP, it must first discover the APs in its vicinity that are potentially available. One method for performing this search is the passive scanning mode. In passive scanning, the STA listens for beacon packets sent by neighboring APs, which are typically sent approximately every 100 ms. The STA listens on each transmission channel, and changes channels at a set interval. Once the STA has listened to a beacon and decoded the information contained in the beacon, it may initiate a procedure for reassociating to the neighboring AP.
The passive scanning mode suffers from a drawback, because the neighboring APs may be operating on different channels. Thus, to listen to the beacon, the STA has to tune to a different channel than the one it is currently operating on with its associated AP. Since the STA generally does not know about the timing of beacon transmissions from other APs, it may need to dwell on each channel for a relatively long time (>100 ms) to ensure that it has received the beacons from all neighboring APs. While it is listening on other channels, the STA cannot receive packets from its associated AP and its communications are thus disrupted. Such disruption may adversely and significantly impact the quality of service perceived by the user.